Most of us, when we travel, put appointments we will have while on the road into our calendars. And we usually enter them in local time. ie. if I have a 1pm appointment in New York, I set it for 1pm not 10am in my Pacific home time zone. While some calendar programs let you specify the time zone for an event, most people don't, and many people also don't change the time zone when they cross a border, at least not right away. (I presume that some cell phone PDAs pick up the new time from the cell network and import it into the PDA, if the network provides that.) Many PDAs don't really even let you set the time zone, just the time.
Here's an idea that's simple for the user. Most people put their flights into their calendars. In fact, most of the airline web sites now let you download your flight details right into your calendar. Those flight details include flight times and the airport codes.
So the calendar software should notice the flight, look up the destination airport code, and trigger a time zone change during the flight. This would also let the flight duration look correct in the calendar view window, though it would mean some "days" would be longer than others, and hours would repeat or be missing in the display.
You could also manually enter magic entries like "TZ to PST" or similar which the calendar could understand as a command to change the zone at that time.
Of course, I could go on many long rants about the things lacking from current calendar software, and perhaps at some point I will, but this one struck me as interesting because, in the downloaded case, the UI for the user is close to invisible, and I always like that.
It becomes important when we start importing our "presence" from our calendar, or get alerts from our devices about events, we don't want these things to trigger in the wrong time zone.